Measuring multiple cosmogenic nuclides in glacial cobbles sheds light on Greenland Ice Sheet processes

• Published in: Earth and Planetary Science Letters
• Main Authors: Corbett, Lee B., Bierman, Paul R., Neumann, Thomas A., Graly, Joseph A., Shakun, Jeremy D., Goehring, Brent M., Hidy, Alan J., Caffee, Marc W.
• Language: unknown
• Published: 2021
• Subjects: 58 GEOSCIENCES; Greenland; Ice Sheet
• Online Access: http://www.osti.gov/servlets/purl/1738905; https://www.osti.gov/biblio/1738905; https://doi.org/10.1016/j.epsl.2020.116673

The behavior of the Greenland Ice Sheet during the Pleistocene remains uncertain due to the paucity of evidence predating the Last Glacial Maximum. In this study, we employ a novel approach, cosmogenic nuclide analysis of individual subglacially-derived cobbles, which allows us to make inferences about ice sheet processes and subglacial erosion. From three locations in western Greenland, we collected 86 cobbles from the current ice sheet margin and nine cobbles exposed on the modern proglacial land surface. We measured the concentration of in situ 10 Be in all cobbles (n = 95) and 26 Al and 14 C in a subset (n = 14). Cobbles deposited during Holocene retreat have 10 Be exposure ages generally consistent with the timing of ice retreat determined by other methods. Conversely, most of the 86 subglacial cobbles contain very low concentrations of 10 Be (median 1.0×10 3 atoms g –1 ), although several have ~10 4 and one has ~10 5 atoms g –1 . The low concentrations of 10 Be in most subglacial cobbles imply that their source areas under the Greenland Ice Sheet are deeply eroded, preserving minimal evidence of surface or near-surface exposure. The presence of measurable 14 C in ten of the cobbles requires that they experienced cosmogenic nuclide production within the past ~30 ka; however, 14 C/ 10 Be ratios of ~6 suggest that nuclide production occurred during shielding by overlying material. Only two of the 86 subglacial cobbles definitively have cosmogenic nuclide concentrations consistent with prior surface exposure. Overall, isotopic analysis of subglacial cobbles indicates that much of western Greenland's subglacial landscape is characterized by deep erosion and minimal subaerial exposure.